This invention relates to a sheet conveyor applied to an image recording apparatus such as a printer, a typewriter, a laser beam printer or an electrophotographic copying apparatus. More particularly, the present invention relates to a sheet conveyor for engaging an engaging member with openings formed in a sheet and conveying the sheet by displacing the engaging member.
2. Related Background Art
Various structures have been proposed as paper feeding mechanisms used, for example, in printers, and one of them is a pin feed mechanism.
This pin feed mechanism has a structure in which a pin wheel rotatable with a platen is provided near the platen and pins projecting from the pin wheel are fitted in through-holes formed along the opposite side edges of recording paper at the same pitch as the interval between the pins, thereby effecting paper feed.
This pin feed mechanism is generally classified into two types, i.e., the pull type as shown in FIG. 1 of the accompanying drawings and the push-in type as shown in FIG. 2 of the accompanying drawings.
In a pull type mechanism recording paper 1 is guided along a paper guide 2 and between a platen 3 and a recording head 4, the recording paper 1 is urged against the platen 3 by a bail roller 6, the pins 5aof a pin wheel 5 are fitted into the feed holes of the recording paper and the recording paper is pulled and thereby fed. Designated by 4a is a recording portion which effects recording on the recording paper 1.
Denoted by 7 in FIG. 1 is a carriage carrying the recording head 4 thereon, and recording is effected while the carriage 7 is moved along a guide shaft 8. When recording of one line is completed, the pin wheel 5 is rotated by a predetermined angle by the drive force from a drive source (not shown), and the recording paper is fed by an amount corresponding to one line. Where such a pull type pin wheel mechanism is used, if an attempt is made to feed the recording paper in reverse, there is the possibility of the feeding of the recording paper becoming unstable.
Also, the distance from the upper end of the recording paper to the recording starting position is required to be equal to at least the distance from the recording portion to the pin wheel, and this leads to the problem that the recording paper cannot be used effectively.
In such a pull type pin feed mechanism, the pin wheel is situated on the outlet side of the recording paper, whereas in the push-in type pin feed mechanism, the pin wheel is situated on the recording paper side as shown in FIG. 2.
In FIG. 2, portions identical or corresponding to those in FIG. 1 are given similar reference characters and need not be described.
In a push-in type pin feed mechanism, the pin wheel is situated on the inlet side of the recording paper and therefore, the recording starting position of the recording paper is at a very short distance from the upper end of the recording paper and thus, the recording paper can be used effectively. However, in a push-in type pin feed mechanism, the recording paper is pushed in and paper feed is effected by resorting to the rigidity of the recording paper and therefore, if the distance from the pin wheel to the recording position is long, there is the possibility of the conveyance of the recording paper becoming unstable.
Also, in the push-in type pin feed mechanism, the recording paper 1 is urged against the platen 3 by the bail roller 6, and the friction feed of the platen 3 rotated for the conveyance of the recording paper is combined with the friction of the conveyance force of the push-in by the pin wheel 5, but adjustment of the strengths of the two conveying forces is difficult and also is liable to be affected by the kind, temperature, etc. of the recording paper, and this also leads to the possibility of the conveyance of the recording paper becoming unstable.
Also, both the pull type pin feed mechanism and the push-in type pin feed mechanism require a considerable installation space, and this hinders to the compactness of the recording apparatus.
To solve such problems, a structure as shown in FIGS. 3 and 4 of the accompanying drawings has been proposed.
Designated by 9 in these Figures is a pin wheel to which a rotational force is transmitted by a drive source, not shown, through a feed shaft 10. A plurality of through-holes 9a are formed in the pin wheel 9 at predetermined angular intervals circumferentially thereof, and a pin 11 is slidably fitted in each of the through-holes 9a. A fixed cam 12 is disposed on the outer side of the pin wheel 9, as shown in FIG. 4, and a rail portion 12a is provided on the inner side of the fixed cam 12.
Two projections 11a and 11a provided on the sides of each pin 11 sandwich the rail portion 12a therebetween, and with rotation of the pin wheel 9, the pins 11 move into and out of the pin wheel 9.
This reciprocation of pins 11 is determined by the shape of the rail portion 12a.
In the example shown, when the pins come to the recording head 4 side, the pins move into the pin wheel 9. Also, the pins are adapted to protrude from the pin wheel when they are at a position far from the recording head 4. Thus, under the pin wheel 9, the recording paper 1 is nipped between the pin wheel 9 and the paper guide 2 so that the feed holes do not come off the pins 11. Above the pin wheel 9, the recording paper 1 is nipped between a paper keeper 13 and the pin wheel 9 so that the feed holes do not come off the pins 11. It is for the purpose of avoiding the interference with the recording head 4 that on the recording head side, the pins 11 move into the pin wheel.
As shown in FIG. 4, a pin feed body 14 is disposed on the inner side of the pin wheel 9, and with the aforementioned fixed cam 12, it is prevented from rotating by a pin feed guide shaft 15.
The pin wheel 9 is disposed at each end of the platen 3 and therefore, if such a structure is adopted, pin feed is effected near the recording position and the mating of the feed holes of the recording paper and the pins is possible upstream and downstream of the recording position. Also, both during forward rotation and during reverse rotation, stable conveyance of the recording paper can be realized, and the distance from the upper end of the recording paper to the recording starting position becomes shorter and the recording paper can be utilized effectively.
Also, the pin wheels can be disposed coaxially with the platen and therefore, the installation space can be reduced and compactness of the device can be realized.
However, in spite of the above-described advantages, the pin feed mechanism in which the pins move into and out of the pin wheel suffers from the following problem.
That is, the pins protruding under the pin wheel 9 are situated at a position which cannot be seen by the user. Therefore, the user must fit the pins into the feed holes when he or she sets recording paper, but it is difficult and cumbersome to do it at a position which cannot be seen.